The N-propionylated group B meningococcal polysaccharide mimics a unique bactericidal epitope on the surface of group B meningococci and Escherichia coli K1. This was confirmed when both the above organisms were able to absorb the bactericidal antibodies from a mouse-anti-N-propionylated group B meningococcal polysaccharide-tetanus toxoid conjugate serum. By using affinity columns it was possible to divide the conjugate antiserum into three distinct populations of both group B polysaccharide cross-reactive and non-cross-reactive antibodies, one of which contained most of the bactericidal activity. The cross-reactive (IgG1) antibodies were absorbed by an affinity column in which the group B polysaccharide was linked to the solid support by a long spacer arm, thereby isolating a population of non-cross-reactive (IgG1) antibodies. Surprisingly the above column also retained another population of non-cross-reactive (IgG2a) and (IgG2b) antibodies which contained most of the bactericidal activity. These latter antibodies were not absorbed by a similar group B polysaccharide-affinity column in which a short spacer arm was employed. Thus the above experiments not only effected a separation of highly bactericidal antibodies but also provided evidence that the long spacer arm is functional in the binding of the bactericidal antibodies to the affinity column. This indicates that the bactericidal epitope is mimicked by the group B polysaccharide in the presence of the long spacer arm, which supports the hypothesis that the epitope is polysaccharide-associated and is probably intermolecular in nature.